A few terror suspects have managed to maintain contact with terrorist groups despite being under control orders which include restrictions that amount to "virtual house arrest", the government's official watchdog on the terror laws warned tonight.

Lord Carlile of Berriew QC said that ­classified material he had seen justified the conclusion that there were "a few controlees" who had managed to remain in contact with other terrorist individuals and groups, and who remained ­determined to act in the future.

It is thought that despite tight surveillance these terror suspects managed to maintain contact through illicit use of mobile phones and unsupervised meetings.

The Conservatives said last night that the disclosure was extraordinary. "We told them that this system wouldn't work properly, but they just wouldn't listen," said the shadow home secretary, Chris Grayling.

The anti-terror watchdog also warned against using the control order regime to deal with any former detainees returned from Guantánamo Bay, saying it should not be used as a routine form of control against people who are seen as potentially troublesome. "It is over-simplistic to assume that they would be appropriate, acceptable, practicable or even lawful against a group of people simply because they had been detained elsewhere, under a foreign and unusual jurisdiction," said Carlile.

He also strongly recommended to the home secretary, Jacqui Smith, that there should be a statutory presumption against a control order being extended beyond two years, save for genuinely exceptional circumstances. This recommendation was rejected last year by the Home Office, but Carlile is pressing again this year. He also renewed his call for the use of intercept evidence in criminal trials, adding that this had the potential to reduce the number of control orders – although it was far from certain.

His annual report on the operation of the control order system confirmed that there are 15 control orders in force and that there have been 23 others who have at some point been subject to their restrictions. A total of seven terror suspects have absconded while under a control order. Six others have been deported, four had their orders revoked and three others have been quashed by the court of appeal.

Control orders are imposed by order of the home secretary, based on intelligence information against terror suspects, when there is insufficient evidence to mount a criminal prosecution. The decision to impose a control order has to be ratified by judges sitting as the special immigration appeals commission.

Among the restrictions that can be imposed are curfews of up to 13 hours enforced by an electronic tag, restrictions on the use of mobile phones and the internet, vetting of all visitors and meetings, and restrictions on the suspect's movements. They need to be renewed every 12 months.

The control order regime was introduced four years ago to replace the system of indefinite detention of international terror suspects in Belmarsh prison, after it was ruled discriminatory and disproportionate by the House of Lords.

Carlile revealed that three of the current 15 suspects subject to a control order face trials from next month for breaching the terms of their orders.

In case A, the ­suspect is alleged to have breached his obligations involving communications and the boundary of the area where he is supposed to remain. His trial cannot take place until the outcome of a court of appeal case on whether he can face a fair trial.

Carlile said he had reviewed the material in each case where a control order was issued in 2008 and said he would have reached the same decision as the home secretary each time. He also confirmed that a control order review group meets to discuss the restrictions in each case.

They enable the home secretary to impose a wide range of restrictions on any person, based on confidential intelligence, she suspects of involvement in terrorism-related activity, whether a UK national or not, and whether the terrorist activity is domestic or international